In a Orthogonal Frequency Division Multiple Access (OFDMA) system of multi-carrier scheme, resources are allocated in units of subchannels, each including subcarriers. A plurality of users separately hare all subcarriers, so multi-user diversity gain is obtained in frequency region. In an OFDMA broadband mobile Internet access system such as WiBro, all cells reuse the same frequency and an Adaptive Modulation & Coding (AMC) scheme is applied according to received signal strength and interference between neighbor cells due to reuse of the same frequency, thereby maximizing throughput.
However, in such a system having a Frequency reuse Factor (FRF) of 1, inter-cell interference is severe and throughput reduction is inevitable at edges (i.e., boundaries) of cells or sectors. This may cause service outage. In a method for improving performance at cell edges when a frequency reuse factor of 1 is used, all subcarriers are orthogonally divided into a number of frequency partitions and the frequency partitions are appropriately arranged in cells such that a specific frequency partition is not used or is used at low power in each cell, thereby reducing interference of the same channel between neighbor cells. This method is referred to as a Fractional Frequency Reuse (FFR) scheme.
In order to apply FFR to an actual system, a band to be used in each cell may be determined based on frequency partitions arranged in the cell according to location information of each Mobile Station (MS). In actual situations, a signal to interference ratio may be dynamically reflected in determining which frequency partitions are to be used for each cell among a band allocated to the cell since the signal to interference ratio constantly varies in the same band due to movement of the MS, fading, etc.
In order to dynamically use resources taking into consideration the signal to interference ratio or the like when partial frequency partitions have been allocated to each cell as described above, it is necessary to take into consideration fairness between users in addition to the given Frequency Reuse Factor (FRF).
When all subcarriers are orthogonally divided into a number of frequency partitions in the OFDMA system as described above, various types of FFR schemes may be taken into consideration to allow cells to share these frequency partitions. The following is a description of the concept and characteristics of such FFR schemes.
As the FRF approaches 1, inter-cell interference due to use of the same channel at cell edges may increase, thereby reducing communication performance, although the size of a band that is available in the cell increases. On the other hand, as the FRF increases, the size of the available band decreases, thereby reducing band efficiency, although inter-cell interference due to use of the same channel decreases.
FIG. 1 illustrates an example FFR scheme.
Referring to FIG. 1, FFR is a method for increasing cell capacity and user Quality of Service (QoS). In the FFR scheme, services are provided to users located near a Base Station (BS) using a frequency reuse factor (FRF) of 1 (i.e., using all subcarriers) to maximize total cell capacity since the level of inter-cell interference will be relatively low for users located near the BS from the viewpoint of the entire cell. In the case where the FRF 1 is used, a FRF of 3 is used for cell-edge users expected to undergo a high inter-cell interference level (i.e., not all subcarriers are used but instead part of the bands of FRF 3 is used for each sector), thereby reducing inter-cell interference to provide high quality services.
FFR is classified into hard FFR in which frequency bands used by cell-edge users of other cells are not used and soft FFR in which such frequency bands are also used with restriction of power and specific conditions.
The soft FFR scheme is a general concept including hard FFR. In the soft FFR scheme, neighbor cells set different transmission power levels for each frequency partition, thereby increasing overall cell capacity. Here, the soft FFR scheme becomes a hard FFR scheme if transmission power is set to 0.
FIG. 2 illustrates example hard and soft FFR schemes.
Referring to FIG. 2, in the case of the hard FFR scheme, only specific frequency bands are used among frequency bands of FFR ⅓ in each cell. On the other hand, it can be seen that, in the case of the soft FFR scheme, all frequency bands of FFR ⅓ are at different power levels in each cell. For example, the power level of the same frequency band of FFR ⅓ may be different for each cell. In addition, each cell may have different power levels for the frequency bands of FFR ⅓.
When the FFR scheme is applied, each frequency band generally has a different power level as shown in FIG. 2. The present invention suggests how to set a power level for each FFR group or each frequency partition and how to perform signaling in downlink and uplink.